Make the cake: In a large bowl using a hand-held mixer or stand mixer fitted with a paddle attachment, cream the softened butter for about 1 minute on medium speed. Get it nice and smooth, then add the sugar on medium speed until fluffy and light in color. Beat in egg, egg yolk, and vanilla. Scrape down the sides as needed.

In a medium bowl, whisk together the flour, salt, baking powder, baking soda, and cornstarch. With the mixer running on low speed, slowly add the dry ingredients to the wet ingredients in 3 different parts. The dough is quite thick. Once combined, gently fold in the white chocolate chips and 1/2 cup sprinkles using a rubber spatula (or by mixing on low speed).

Press the cookie dough evenly into the prepared pan. Bake for 25-30 minutes or until very lightly browned on top. You want the cookie cake to be extra soft in the center, so careful not to overbake. Allow the cake to cool completely on a wire rack at room temperature before frosting. The center will slightly sink - that's normal.

Make the frosting: With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy - about 2 minutes. Add confectioners' sugar, cream, vanilla and almond extracts with the mixer running on low. Increase to high speed and beat for 3 full minutes. Add more confectioners' sugar if frosting is too thin or more cream if frosting is too thick. Add a pinch of salt if frosting is too sweet. I usually add anywhere between 1/8 - 1/4 teaspoon of salt.

Frost the cake as desired, I used a Wilton 1M piping tip to decorate the edges. Using a very sharp knife, cut into slices and serve. This cake goes wonderfully with vanilla ice cream! Store cake in an airtight container at room temperature for up to 2 days and in the refrigerator up to 5.

Recipe Notes

Room temperature egg and egg yolk preferred. To bring the egg to room temperature quickly, place in a warm glass of water for about 10 minutes as you get the other ingredients ready. Or just set out when you set out the butter to soften 1 hour ahead of time. Use regular sprinkles (aka "jimmies") in the cookie cake base. Nonpareils (the little balls) will bleed their color and turn the sugar cookie dough green/purple/brown. Feel free to decorate the top of the cake/frosting with nonpareils, though. I also sprinkled some nonpareils on top of the pressed-in dough before baking. Cream or half-and-half is preferred for creamiest, fluffiest frosting. Keeping that in mind, milk would be an OK substitute if you do not have either on hand. The almond extract is optional but highly encouraged; I love its flavor combined with the vanilla frosting and buttery sugar cookie cake!